Semiconductor storage location

ABSTRACT

In a semiconductor memory cell, in particular in a DRAM memory cell array, having a selection transistor ( 12 ) and a storage capacitor ( 14 ), in which the storage capacitor ( 14 ) has a first ( 16 ) and a second ( 18 ) capacitor electrode, the first capacitor electrode ( 16 ) is connected to a read-out line ( 22 ) via the selection transistor ( 12 ), and in which a control terminal ( 32 ) of the selection transistor ( 12 ) is connected to a control line ( 24 ), as a special feature a layer of a superionic conductor ( 20 ) is arranged between the first and second capacitor electrodes ( 16, 18 ) of the storage capacitor ( 14 ). The high conductivity of the superionic conductor ( 20 ) for ions in association with a negligible electron conductivity allows extremely high capacitances to be produced in a small space.

[0001] The invention relates to a semiconductor memory cell, in particular in a DRAM memory cell array, having a selection transistor and a storage capacitor, in which the storage capacitor has a first and a second capacitor electrode, the first capacitor electrode is connected to a read-out line via the selection transistor, and in which a control terminal of the selection transistor is connected to a control line.

[0002] Memory cells of this type are used for example in dynamic random access memories (DRAMs). A DRAM chip contains a matrix of memory cells which are arranged in the form of rows and columns and are addressed by word lines as control lines and bit lines as read-out lines. The read-out of data from the memory cells, or the writing of data to the memory cells, is realized by the activation of suitable word lines and bit lines.

[0003] Each of the memory cells contains a capacitor for the purpose of charge storage, the charge state in the capacitor representing a data bit. The memory cell usually further contains a transistor connected to a capacitor. The transistor has two diffusion regions separated from one another by a channel which is controlled by a gate as control terminal. Depending on the direction of the current flow, one diffusion region is designated as the drain and the other as the source. The drain region is connected to the bit line, the source region is connected to the capacitor and the gate is connected to the word line. By the application of suitable voltages to the gate, the transistor is controlled in such a way that a current flow between the drain region and the source region through the channel is switched on and off.

[0004] The charge stored in the capacitor decreases with time on account of leakage currents. Before the charge has decreased to an indeterminate level below a threshold value, the storage capacitor must be refreshed. For this reason, these memory cells are referred to as dynamic RAM (DRAM). Such a memory cell having the features of the preamble of claim 1 is disclosed for example in the Patent Specification U.S. Pat. No. 5,867,420.

[0005] The central problem in the case of the known DRAM variants is that of producing a sufficiently large capacitance of the capacitor. This problem area will be aggravated in future by the advancing miniaturization of semiconductor components. The continuous increase in the integration density means that the area available per memory cell and thus the capacitance of the capacitor decrease ever further. An excessively small capacitance of the capacitor can adversely affect the functionality and usability of the memory device since an excessively small quantity of charge is stored on it.

[0006] By way of example, the sense amplifiers connected to the bit line require a sufficiently high signal level for a reliable read-out of the information held in the memory cell. The ratio of the storage capacitance to the bit line capacitance is crucial in determining the signal level. If the storage capacitance is too low, this ratio may be too small for the generation of an adequate signal.

[0007] A smaller storage capacitance likewise requires a higher refresh frequency since the quantity of charge stored in the capacitor is limited by its capacitance and additionally decreases due to leakage occurrence. If the quantity of charge in the storage capacitor falls below a minimum quantity of charge, then it is no longer possible to read out the information stored in it by means of the connected sense amplifiers, the information is lost and read errors occur.

[0008] According to a rule of thumb, the storage capacitance should be at least about 35 ff in order to obtain a sufficiently large read signal and sufficient insensitivity to alpha radiation. With the use of a dielectric 10 nm thick made of SiO₂ with a dielectric constant (DC) of ε_(r)=4, this requires a capacitor area of about 10 μm². However, even with a 4M DRAM, there is already less area than this available for the entire memory cell, thereby ruling out a purely planar arrangement of the capacitor.

[0009] It has therefore become necessary, in order to obtain sufficient storage capacitance for the capacitor layout, to utilize the third dimension, for example by configuring the capacitor as a trench capacitor or stacked capacitor. With further miniaturization, the smaller area available can then be compensated for by means of an increase in the capacitance through the use of deeper trenches or higher stacks.

[0010] Another approach consists in using materials having a larger dielectric constant. By way of example, Si₃N₄ with a DC of 7 is used in particular in the form of ONO (oxide-nitride-oxide) and NO (nitride-oxide) sandwiches. In this case, a thermal oxide having a thickness of 2-3 nm is grown on the silicon, for example, in order to ensure a low interfacial state density. A silicon nitride layer having a thickness of 7-8 nm is then deposited and subsequently oxidized in order to obtain a second oxide layer having a thickness of 2-3 nm. This second oxide layer serves for preventing the tunneling of charge carriers by means of a high energy barrier.

[0011] Even the use of materials having an even higher DC, such as, for example, tantalum oxide (Ta₂O₅) or barium strontium titanate (BST), is possible although not unproblematic in terms of process engineering. With this possibility, the storage capacitance that can be achieved is upwardly limited by the dielectric constant and the thickness of the dielectric at which the latter still effects insulation.

[0012] This forms the starting point for the invention, the invention, as it is characterized in the claims, is based on the object of specifying a memory cell of the generic type whose storage capacitor has a high storage capacitance per area and thus enables a small structural form.

[0013] This object is achieved by means of the memory cell having the features of claim 1. Preferred refinements emerge from the subclaims.

[0014] According to the invention, in the case of a semiconductor memory cell of the type mentioned in the introduction, a layer of a superionic conductor is arranged between the first and second capacitance electrodes of the storage capacitor.

[0015] The invention is thus based on the concept of providing a layer of a superionic conductor, instead of a dielectric, between the two capacitor electrodes. While the superionic conductor on the one hand has a high conductivity for ions, its electron conductivity can be so low that it practically blocks the electron DC current. On the other hand, owing to the high ion conductivity, the total capacitance of the capacitor is not determined by the bulk capacitance of the ionic conductor but rather only by the interface capacitances between ionic conductor and capacitor electrodes. In this way, it is possible to produce extremely high capacitances in a small space.

[0016] According to the statements above, it is preferable if the electron conductivity of the superionic conductor layer is negligibly small. In the present context, this means that the electron conductivity is so small that the ionic conductor layer acts as an insulator with regard to the electron DC current under the customary operating conditions of a semiconductor memory cell.

[0017] In a preferred refinement, the superionic conductor layer essentially comprises a tysonite, in particular (Ca, La, RE)F₃. In this case, RE denotes a rare earth metal, that is to say an element from the group SC, Y, La, Ce, Fr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. In this class of ionic conductors, fluorine ions are responsible for the charged transport.

[0018] In this case, the superionic conductor layer is preferably formed from doped LaF₃. Particularly good results are obtained in the case of heterovalent doping with SrF₂, the proportion of SrF₂ expediently being from 0.1 to 10% by weight. A proportion of from 1 to 5% by weight is preferred, and a proportion of about 2% by weight is especially preferred. As a result of the doping, the ion conductivity of LaF₃ can be increased again by a plurality of orders of magnitude.

[0019] In one refinement of the invention, the storage capacitor of the memory cell is formed as a trench capacitor.

[0020] In another preferred refinement of the invention, the storage capacitor of the memory cell is formed as a stacked capacitor.

[0021] Further advantageous refinements, features and details of the invention emerge from the dependent claims, the description of the exemplary embodiments and the drawings.

[0022] The invention will be explained in more detail below using an exemplary embodiment in connection with the drawings. Only the elements which are essential for understanding the invention are illustrated in each case. In the figures,

[0023]FIG. 1 shows a diagrammatic illustration of a memory cell of a memory cell array according to an exemplary embodiment of the invention;

[0024]FIG. 2 shows an equivalent circuit diagram for the storage capacitor of FIG. 1;

[0025]FIG. 3 shows a DRAM memory cell according to one embodiment of the invention.

[0026]FIG. 1 shows a memory cell 10 of a larger memory cell array. The memory cell 10 contains a selection transistor 12 and a storage capacitor 14. The first capacitor electrode 16 of the storage capacitor 14 is connected to the bit line 22 via the selection transistor 12, and the gate 32 of the selection transistor 12 is connected to the word line 24.

[0027] By the application of a corresponding voltage to the gate 32, the transistor 12 is turned on, and the charge stored in the capacitor 14 flows onto the bit line 22, where it causes a voltage level change which is amplified by a sense amplifier (not shown) assigned to the bit line 22, so that it is available for read-out. After the read-out operation, the information bit is written back to the capacitor 14 again.

[0028] A thin layer of a superionic conductor, in the exemplary embodiment a thin layer 20 made of LaF₃ doped with 2% by weight of SrF₂, is arranged between the capacitor electrodes or capacitor contacts 16, 18. This layer 20 combines a high ion conductivity with a negligible electron conductivity.

[0029] In the exemplary embodiment, the thin layer is fabricated by coevaporation of LaF₃ and SrF₂ in vacuum at a pressure below 5×10⁻⁶ mbar and a substrate temperature of about 500° C.

[0030] With this composition of the ionic conductor, even with a layer thickness of 240 nm a capacitance of 4 nF/mm² could already be achieved, which corresponds to an apparent dielectric constant of about 100.

[0031] The manifestation of the high capacitance of the capacitor 14 with superionic conductor layer 20 will now be explained in connection with the equivalent circuit diagram of FIG. 2.

[0032] In this case, the bulk capacitance 52 of the ionic conductor layer 20 and the interface capacitances 50 and 56 of the ionic conductor with respect to the metallic or semiconducting capacitor plates 16, 18 are influential as quantities to be taken into account. On account of the high ion conductivity, the capacitance 52 is practically bridged via the small resistor 54, so that the total capacitance is essentially determined only by the interface capacitances 50 and 56.

[0033] A concrete exemplary embodiment of a memory cell having a superionic conductor layer in a trench capacitor is shown in cross section in FIG. 3. In this case, doping regions 30, 34 are formed in the silicon substrate 40, said doping regions forming drain and source of the selection transistor 12. The gate 32 of the transistor is connected to the word line 24, which extends perpendicularly to the plane of the drawing in FIG. 3.

[0034] The bit line 22 is connected to the drain doping region 30 of the transistor via a contact 26. The source doping region 34 produces the connection to the trench capacitor 14.

[0035] One of the two capacitor electrodes is formed by a conductive trench filling 16, for example made of doped poly-Si. The counter electrode is formed by the buried doping region 18, which is electrically connected via a buried well (not illustrated) to adjacent memory cells and a voltage source.

[0036] For the insulation of the doping regions 23 and 18, an insulation collar 36 is situated in the upper part of the trench.

[0037] Instead of the dielectric that is usually provided, in the exemplary embodiment a superionic conductor layer 20 is arranged between the two capacitor electrodes 16, 18, the composition of which layer may correspond to that described above.

[0038] The high apparent dielectric constant of the material in connection with a small layer thickness and the configuration of the capacitor as a trench capacitor allows an extremely high capacitance to be produced in a very small space and thus a memory cell to be produced which can be miniaturized within a wide range. 

1. A semiconductor memory cell, in particular in a DRAM memory cell array, having a selection transistor (12) and a storage capacitor (14), in which the storage capacitor (14) has a first (16) and a second (18) capacitor electrode, the first capacitor electrode (16) is connected to a read-out line (22) via the selection transistor (12), and in which a control terminal (32) of the selection transistor (12) is connected to a control line (24), characterized in that a layer of a superionic conductor is arranged between the first and second capacitor electrodes (16, 18) of the storage capacitor (14).
 2. The semiconductor memory cell as claimed in claim 1, in which the superionic conductor layer (20) has a negligible electron conductivity.
 3. The semiconductor memory cell as claimed in claim 1 or 2, in which the superionic conductor layer (20) essentially comprises a tysonite, in particular (Ca, La, RE)F₃, where RE denotes a rare earth.
 4. The semiconductor memory cell as claimed in one of the preceding claims, in which the superionic conductor layer (20) is formed from doped LAF₃.
 5. The semiconductor memory cell as claimed in claim 4, in which the superionic conductor layer (20) is doped with SrF₂, preferably with an SrF₂ proportion of from 0.1 to 10% by weight, particularly preferably from 1 to 5% by weight, especially preferably of about 2% by weight.
 6. The semiconductor memory cell as claimed in one of the preceding claims, in which the storage capacitor (14) is formed as a trench capacitor.
 7. The semiconductor memory cell as claimed in one of claims 1 to 5, in which the storage capacitor (14) is formed as a stacked capacitor. 